The present invention relates to combustion controllers for internal combustion engines that shift combustion modes in accordance with the operating conditions of the engine.
In a typical automotive engine, fuel is injected into an intake port by a fuel injector to charge the associated combustion chamber with a homogeneous mixture of fuel and air. An air intake passage is opened and closed by a throttle valve, which is operated in cooperation with an acceleration pedal. The opening of the throttle valve adjusts the intake air amount (and ultimately the amount of homogeneously mixed air and fuel) that is supplied to the combustion chambers of the engine. This controls engine power.
The combustion of the air fuel mixture in such homogeneous state is normally referred to as homogeneous charge combustion. However, in engines that perform homogeneous combustion, the throttling action of the throttle valve decreases the pressure in the intake passage. This increases energy loss due to pumping (pumping loss) when the air fuel mixture is drawn into the combustion chambers from the intake passage and thus decreases the efficiency of the engine. This problem can be dealt with by direct injection. In an engine that performs direct injection, the throttle valve is opened wide and fuel is injected directly into each combustion chamber. The air drawn into each combustion chamber is swirled and mixed with atomized fuel to form a stratum of a stoichiometric air-fuel mixture near the spark plug. Such stratified charge combustion increases the overall air-fuel ratio of the mixture that is burned. This improves fuel efficiency.
Stratified charge combustion is the optimal combustion mode if the engine is in a low speed range or intermediate speed range, and the amount of fuel injection is thus small. In comparison, homogeneous charge combustion is the optimal combustion mode if the engine is in a high speed range, and the amount of fuel injection is thus great. Therefore, it is beneficial for an engine to perform both stratified charge combustion and homogeneous charge combustion.
Japanese Unexamined Patent Publication No. 5-52145 describes an engine that shifts combustion modes (or fuel injection modes) between stratified charge combustion, homogeneous charge combustion, and semi-stratified charge combustion. Stratified charge combustion is performed when the load applied to the engine is light. In this mode, fuel is injected during the compression stroke. If a heavy load is applied to the engine, homogeneous charge combustion is performed. In this mode, fuel is injected during the intake stroke. If an intermediate load is applied to the engine, semi-stratified charge combustion is performed. In this mode, fuel is injected twice, once during the intake stroke and once during the compression stroke.
The shifting of the combustion mode, or the fuel injection mode, results in certain problems. For example, when the combustion mode is shifted from stratified charge combustion to homogeneous charge combustion, the amount of fuel injection is increased. Furthermore, the intake air is throttled by the throttle valve. This results in energy loss (pumping loss) and decreases the engine torque in a sudden manner. It is preferable for the torque to increase gradually as the engine load increases. However, if the fuel injection is determined in accordance with changes in the engine load without taking pumping losses into consideration, sudden torque fluctuations will occur when switching the combustion mode.